docs: update documentation with chat enhancements

Document three recent chat improvements:

1. Real-time Active Users Fix (dd31761)
   - Fixed bug where users list didn't update when new users joined
   - Rewrote getAllDisplayUsers() to prioritize Socket.IO data

2. Message Length Limits (4a91a10)
   - Added 2000 character limit with visual counter
   - Backend + frontend validation

3. Spam Protection System (ace3311)
   - Rate limiting: 10 messages per minute
   - Duplicate detection within 60 second window
   - Profanity filter with Polish + English words

Updated:
- README.md: Added chat features to Events & Chat section
- SESSION_CONTEXT.md: New "Chat Enhancements" section
- COMPLETED.md: Comprehensive entry with problem/solution/impact
- Last updated dates: 2025-12-03

docs/archive/COMPLETED.md

@@ -1436,7 +1436,153 @@ activityLog({
 
 ---
 
-**Last Updated:** 2025-12-02 (Activity Log System completed)
+## ✅ Chat Enhancements (COMPLETED 2025-12-03)
+
+**Status:** Production Ready
+**Time Spent:** ~2 hours
+**Commits:** 3 commits (`dd31761`, `4a91a10`, `ace3311`)
+**Tests:** Manual testing
+
+### Overview
+Enhanced chat system with real-time updates, message validation, and comprehensive spam protection. Improves UX with instant active user updates, prevents abuse with rate limiting and profanity filtering, and protects against excessive message lengths.
+
+### 1. Real-time Active Users Fix (dd31761)
+
+**Problem:**
+- When new users joined event chat, existing users didn't see them in the active users list without page refresh
+- Socket.IO was working correctly and emitting `active_users` events
+- Frontend was receiving updates but users weren't displayed in UI
+
+**Root Cause:**
+- `getAllDisplayUsers()` function in `EventChatPage.jsx` used `checkedInUsers` (static, loaded once from API) as the base list
+- `activeUsers` (real-time Socket.IO data) was only used to set `isOnline` flag
+- When new user joined: appeared in `activeUsers` but not in `checkedInUsers`, so not displayed
+
+**Solution:**
+- Rewrote `getAllDisplayUsers()` to prioritize real-time data:
+  1. Use `activeUsers` (Socket.IO) as primary source of truth
+  2. Merge with `checkedInUsers` for offline users who checked in
+  3. Enrich Socket.IO data with database data when available
+  4. Sort online users first, offline second
+
+**Files Modified:**
+- `frontend/src/pages/EventChatPage.jsx` - Rewrote `getAllDisplayUsers()` function
+- `frontend/src/hooks/useEventChat.js` - Added debug logging
+- `backend/src/socket/index.js` - Added debug logging for active_users emissions
+
+**Impact:**
+- Instant real-time updates when users join/leave event chat
+- No page refresh required
+- Improved user experience for active chat monitoring
+
+### 2. Message Length Limits (4a91a10)
+
+**Problem:**
+- No character limit for chat messages
+- Database TEXT field could accept very long messages
+- Risk of UI breaking with extremely long messages
+- Poor UX with no feedback about message length
+
+**Solution:**
+- Implemented 2000 character limit across the stack:
+
+**Backend:**
+- Added `MESSAGE_MAX_LENGTH = 2000` constant
+- Validation in `send_event_message` handler
+- Validation in `send_match_message` handler
+- User-friendly error message
+
+**Frontend:**
+- Added `MESSAGE_MAX_LENGTH = 2000` constant
+- `maxLength={MESSAGE_MAX_LENGTH}` attribute on input field
+- Character counter appears at 80% threshold (1600+ characters)
+- Red warning text when at/over limit
+- Submit button disabled when over limit
+
+**Files Modified:**
+- `backend/src/constants/index.js` - Added MESSAGE_MAX_LENGTH constant
+- `backend/src/socket/index.js` - Added validation to both message handlers
+- `frontend/src/constants/index.js` - Added MESSAGE_MAX_LENGTH export
+- `frontend/src/components/chat/ChatInput.jsx` - Enhanced with counter and validation
+
+**Impact:**
+- Prevents database/UI issues from excessively long messages
+- Clear visual feedback for users approaching limit
+- Consistent validation across event and match chat
+- Better UX with proactive character counter
+
+### 3. Spam Protection System (ace3311)
+
+**Problem:**
+- No protection against chat abuse
+- Users could spam messages rapidly
+- No duplicate message detection
+- No profanity filtering (especially important for Polish users)
+
+**Solution:**
+- Created comprehensive `messageValidation.js` middleware with 3 protection mechanisms:
+
+**1. Rate Limiting**
+- 10 messages per minute per user
+- Sliding time window (60 seconds)
+- In-memory tracking with Map: `userId -> timestamps[]`
+- Auto-cleanup of expired timestamps
+
+**2. Duplicate Detection**
+- Prevents identical messages within 1 minute window
+- Tracks last 5 messages per user
+- In-memory tracking with Map: `userId -> [{ content, timestamp }]`
+- Auto-cleanup of old messages
+
+**3. Profanity Filter**
+- Uses `bad-words` library v2.0.0 (CommonJS compatible)
+- English profanity words (built-in)
+- 11 Polish profanity words added
+- User-friendly error message: "Message contains inappropriate language"
+
+**Implementation Details:**
+- Single `validateMessage(userId, content)` function for all checks
+- Returns `{ valid: boolean, error?: string }`
+- Integrated into both `send_event_message` and `send_match_message` handlers
+- Replaced basic validation (empty/length checks only) with comprehensive validation
+- Memory management: Periodic cleanup every 5 minutes
+
+**Technical Note:**
+- Initially installed `bad-words` v3.0.4 (ES module)
+- Backend crashed: "exports is not defined in ES module scope"
+- Fixed by downgrading to v2.0.0 (CommonJS compatible)
+
+**Files Created:**
+- `backend/src/middleware/messageValidation.js` (200 lines)
+
+**Files Modified:**
+- `backend/src/socket/index.js` - Import and integrate validateMessage()
+- `backend/package.json` - Added bad-words@2.0.0
+
+**Impact:**
+- Prevents chat spam and abuse
+- Maintains respectful chat environment
+- Protects against duplicate message flooding
+- Bilingual profanity filtering (English + Polish)
+- User-friendly error messages for each violation type
+- Never blocks legitimate users (reasonable limits)
+
+### Git Commits
+
+1. `dd31761` - fix(chat): real-time active users list updates
+2. `4a91a10` - feat(chat): add 2000 character limit for messages
+3. `ace3311` - feat(chat): implement spam protection and profanity filter
+
+### Access Information
+- Feature available to all logged-in users in event chat and match chat
+- Spam protection runs automatically on every message
+- Rate limit: 10 messages per minute
+- Duplicate window: 60 seconds
+- Character limit: 2000 characters (counter appears at 1600+)
+
+---
+
+**Last Updated:** 2025-12-03 (Chat Enhancements completed)
 **Note:** This file is an archive of completed phases. For current status, see SESSION_CONTEXT.md or TODO.md
 
 **MVP Status:** ✅ 100% Complete - All core features implemented, tested, and production-ready